Pipelines for conveying fluids can be made from a number of different materials, including plastic materials, such as polyvinyl chloride (PVC), and metals, such as steel, brass and aluminum. Each length of such pipe typically has a belled female end and a spigot male end. The joints between sections of pipe are sealed, typically with a gasket. PVC pipe systems have often relied on the “Rieber Joint,” which was developed in the early 1970's by Rieber & Son of Bergen, Norway, to seal the connections. The Rieber Joint has an elastomeric gasket inserted into a groove on the inside of the belled end of a piece of the PVC pipe as the female end is being formed.
It soon became apparent that gaskets made entirely from elastomeric materials were unsuitable for pressure applications. The obvious solution was incorporating a metal ring or band embedded in the elastomeric body. The metal ring or band would secure the position of the gasket during the belling process; retain the gasket in the annular groove of the belled pipe while the pipe joint was assembled; and prevent extrusion of the gasket during pressure testing and pressure surges under operating conditions.
The choice of using a metal ring or flat, hoop-shaped metal band depended on the application of the gasket. The metal ring would typically be completely embedded within the body of the gasket during the injection molding process. A metal band was typically only partially embedded in the elastomeric body of a gasket. As shown in prior art FIG. 1, the flat metal band 1 would be placed as an insert against the cavity wall 2 of the lower mold plate 3 of a mold, with its lower edge resting against the top of core support pin(s) 4, which were circumferentially spaced around the bottom of the cavity wall 2. Elastomeric material would be injected and molded, resulting in a reinforced gasket 5, such as that shown in prior art FIG. 2. U.S. Pat. No. 4,120,521 to Parmann shows a typical gasket produced by the method. The metal band 1 would have an entire face left exposed to the elements and contaminants, which could corrode or degrade the metal band 1. Such a possibility was of concern during storage and during use of the gasket 5 in a pipeline joint.
Embedding the metal band 1 completely within the elastomeric material is clearly desirable in order to improve corrosion resistance. One approach has been to cover the exposed face of the metal band 1 with a thin rubber layer 6 after molding, as shown in prior art FIG. 3. However, because the rubber layer 6 was not molded with the gasket 5, it tended to peel away when the gasket was being installed during belling of the pipe, or during operation of the joint, thus defeating its purpose.
Another approach is shown in U.S. Pat. No. 6,113,159 to Corbett, Jr., which shows a gasket reinforced with a metal band having a circumferential apex. The patent does not disclose how the band is made, nor how the gasket is molded, but the gasket is not currently being manufactured. In any event, after molding, the apex of the metal band is exposed around the outer circumference of the gasket, leading to the same problems described, supra.
Yet another approach is shown in prior art FIGS. 4 and 5. It is known in the art of injection molding to use inserts, which can be molded into an elastomeric material. The inserts are supported by support pins having different geometries and positions, depending on the desired final position of the insert after the molding process is completed. For example, FIG. 4 is similar to FIG. 1, showing a metal band 1 with its lower edge resting against the top of core support pins 4. The outer face of the metal band 1 is spaced slightly apart from the cavity wall 2 of the lower mold plate 3 by axial support pins 7. Alternatively, radial support pins (not shown) could be used. Both types of support pins are known as lateral positioning pins. As shown in FIG. 5, the upper mold plates 8 have been put in place, and the injection molding process had produced a gasket 9 with the metal band 1 embedded therein to a significant degree. This method, which is described in U.S. Patent Publication No. 2009/0200705 A1 to Mora, is well known in the art. While the resulting gasket 9 is an improvement over a gasket with an exposed face, the metal band 1 is not entirely encapsulated. As shown in FIG. 6, the ends of the axial pins leave small indentations 10 in the surface of the body of the gasket 9, exposing a significant portion of the metal band 1 to corrosion. Further, the additional axial support pins 7 used means that more care is required when the metal band 1 is inserted, and the number of pins that could break or deteriorate during the molding process is increased.
There exists a need for an improved gasket with an encapsulated metal band, as well as a method for molding such a reinforced gasket.